The present invention relates to an optical sample used for optical writing and/or optical reading beyond the diffraction limit of light using an evanescent field, and also relates to a method for writing and reading information on the optical sample.
In recent years, various high-density recording methods using a near field (evanescent field) have been developed. For instance, a reading or writing method using an evanescent field for an optical memory is disclosed in a magazine xe2x80x9cElectronicsxe2x80x9d, October issue, 1998, pp. 100-102, (published by Ohm Corp.) and in a paper xe2x80x9cApplied Physics Letters, Vol. 73, No. 15, published on Oct. 12, 1998, pp. 2078-2080xe2x80x9d. This method will be explained with reference to FIG. 4.
For example, in an optical disk 101 as an optical memory, a protective layer 112, a mask layer 113, a protective layer 114, a recording layer 115 and a protective layer 116 are deposited in this order on a substrate 111. A phase change material Ge2Sb2Te5 is used for the recording layer 115, and an antimony film is used for the mask layer 113.
For writing and reading of information on the optical disk 101, a laser beam 102 focused by an objective lens (not shown) is applied to the optical disk 101. The antimony film (the mask layer 113) shows a change in the refractive index at a high-temperature portion corresponding to the center of the projected laser spot. As a result, an aperture 103 smaller than the diameter of the spot is produced in the mask layer 113. The aperture 103 is an area of the mask layer 113 which has changed to have a higher transmittance of light. Therefore, for example, during reading of information from the optical disk 101, a mark 104 recorded on the recording layer 115 is read through the aperture 103.
The thickness of the protective layer 114 between the mask layer 113 and the recording layer 115 is set for a distance which allows the evanescent field 105 produced by the aperture 103 to reach the recording layer 115. With this setting, it is possible to record or reproduce a recorded mark of no larger than 100 nm.
However, with the above-mentioned conventional structure, since the aperture 103 is produced using a change of phase of the antimony film from a crystalline state to an amorphous state, the opening and closing speed of the aperture 103 can not be sufficiently increased with respect to a change in temperature. It is thus difficult to increase the opening and closing speed of the aperture 103 in accordance with a rise of the linear velocity, and impossible to perform recording and reproduction of information by a high-speed transfer.
In order to solve the above-mentioned problems, an object of the present invention is to provide an optical sample capable of opening and closing an aperture at a high speed and performing high-speed reading and/or high-speed writing of information by a high-speed transfer, and a method of writing and reading information on the optical sample.
In order to achieve the above-mentioned object, an optical sample of the present invention includes a mask layer made of a material in which a refractive index change region is produced by a chemical change upon application of heat or light, the chemical change being reversible, and a sample layer provided at a position where an evanescent field produced in the refractive index change region when light is applied to the mask layer can reach.
According to this structure, the refractive index change region is produced by a chemical change upon application of heat or light to the mask layer. In the state in which the refractive index change region is produced, when light is applied to the mask layer, an evanescent field that reaches the sample layer is produced in the refractive index change region. Therefore, the information which is, for example, recorded at a high density on the sample layer can be read with high resolution by the mutual function of the evanescent field and the sample layer.
In this reading operation, since the refractive index change region, i.e., an optical aperture is produced in the mask layer by a chemical change, the aperture can be opened and closed at a speed higher than that in a structure where an aperture is formed by using a change of phase of the mask layer from a crystalline state to an amorphous state.
It is thus possible to read the information from the sample layer (for example, a recording layer) of the optical sample (for example, an optical disk as an optical recording medium) at a high speed.
A method of reading information on an optical sample having a mask layer made of a material in which a refractive index change region is produced by a chemical change upon application of heat or light, the chemical change being reversible, and a sample layer provided at a position where an evanescent field which is produced in the refractive index change region when light is applied to the mask layer can reach, includes
producing locally the refractive index change region in the mask layer with application of a light beam to the mask layer, and reading information from the sample layer by a mutual function of the evanescent field produced in the refractive index change region and the sample layer.
According to this structure, the evanescent field that can reach the sample layer is produced in the refractive index change region with application of light to the mask layer, and information which is, for example, recorded at a high density on the sample layer can be read with high resolution by the mutual function of the evanescent field and the sample layer.
Moreover, in this reading operation, since the refractive index change region, i.e., an optical aperture can be opened and closed at a high speed by a chemical change in the mask layer, it is possible to read information from the sample layer (for example, a recording layer) of the optical sample (for example, an optical disk as an optical recording medium) at a high speed.
A method of writing and reading information on an optical sample having a mask layer made of a material in which a refractive index change region is produced by a chemical change upon application of heat or light, the chemical change being reversible, and a sample layer capable of recording information by an optical change, the sample layer being provided at a position where an evanescent field which is produced in the refractive index change region when light is applied to the mask layer can reach, includes
producing locally the refractive index change region in the mask layer with application of a light beam to the mask layer, and writing or reading information on the sample layer by a mutual function of the evanescent field produced in the refractive index change region and the sample layer.
According to this structure, the evanescent field that can reach the sample layer is produced in the refractive index change region when light is applied to the mask layer, and the information which is, for example, recorded at a high density on the sample layer can be read with high resolution by the mutual function of the evanescent field and the sample layer. Furthermore, it is possible to write information on the sample layer.
In the reading operation and writing operation, since the refractive index change region, i.e., an optical aperture can be opened and closed at a high speed by a chemical change in the mask layer, it is possible to write and read information on the sample layer (for example, a recording layer) of the optical sample (for example, an optical disk as an optical recording medium) at a high speed.
For a fuller understanding of the nature and advantages of the invention, reference should be made to the ensuing detailed description taken in conjunction with the accompanying drawings.